The production of pure gases, in particular N2 and O2 from atmospheric air, is an industrial operation performed on a large scale and can make use either of cryogenic processes or of adsorption processes based on the principle of pressure swing adsorption (PSA), that of temperature swing adsorption (TSA) or a combination of the two (PTSA). Furthermore, many gases derived from industrial processes contain large amounts of carbon dioxide which it is often worthwhile purifying.
The production of N2 or O2 from air requires a purification prior to the actual separation step since, when carrying out the cryogenic processes, the water or the carbon dioxide present in the supply air can cause blockages in the equipment due to the fact that these operations are carried out at temperatures very much lower than the freezing points of these impurities. In the adsorption processes, the water and the carbon dioxide are adsorbed more strongly than the nitrogen and lead in the long run to poisoning of the adsorbent, with a consequent reduction in its life expectancy.
In these processes, a zeolite of faujasite type (13X, in which the Si/Al ratio is greater than 1.2) is very generally used to eliminate the carbon dioxide, trapping of the water generally being carried out on a bed of alumina placed upstream of the bed of molecular sieves. The regeneration is of PTSA type, i.e. a slight temperature increase to about 115° C. is combined with a decrease in pressure. By these means, the gas arriving at the bed consists only of N2 and O2 with about 1% by volume of argon whose adsorption behaviour can be likened to that of oxygen.
It has been known for a long time that zeolite X is a better adsorbent for carbon dioxide than silica gel or active charcoal (U.S. Pat. No. 2,882,244). That patent also teaches that the selectivity with respect to various adsorbents varies with temperature and pressure.
U.S. Pat. No. 3,885,927 (May 27, 1975) teaches that the adsorption of CO2 can be carried out on a zeolite X exchanged to more than 90% with barium: under these conditions, the CO2 content of the gas does not exceed 1000 ppm and the temperature can be between −40° C. and 50° C.
European patent application No. 88 10 7209.4 (May 5, 1988) teaches that a zeolite X exchanged with strontium can also be used to carry out this purification.
The influence on the adsorption of CO2 of the number of exchangeable cations on the zeolite was studied by Barrer et al. in “Molecular Sieves” (Soc. Chim. Ind., London, 1968), p. 233 and by Coughlan et al. in “J.C.S. Faraday”, 1, 1975, 71, 1809. These studies show that the capacity of the zeolite to adsorb CO2 increases as the Si/Al ratio decreases, to a limit of 1.2, the lower range of which was not explored.
Zeolite X which has an Si/Al ratio close to 1.25 and which is commonly used, is very selective for CO2, and is all the more selective the lower the temperature. At temperatures close to ambient temperatures, the efficacy decreases greatly on account of the competition with nitrogen which is present in much higher molar proportions. The N2/CO2 ratio in ambient air (with CO2˜300/400 vpm) is about 3000. It is thus generally essential to equip the decarbonatation stage with a refrigeration system so as to avoid the temperature increase on adsorption, this increase possibly being large (several tens of degrees) on account of the strong heats of adsorption involved.
U.S. Pat. No. 5,531,808 (Jul. 2, 1996) discloses the teaching that CO2 can be adsorbed very effectively using a zeolite of type X with an Si/Al ratio of less than 1.15. The advantage over “standard” zeolite X lies in the fact that it is no longer necessary to reduce the temperature in the decarbonatation step using a cold unit since the efficacy of the zeolite is such that the selectivity for CO2 over nitrogen remains high even up to 50° C.
It is observed that the capacity of a zeolite NaLSX to adsorb CO2 increases as the level of exchange with sodium increases. However, it is also observed that the gain in efficacy begins to reach a plateau when exchange rates of about 90% are reached, such that there is apparently no further advantage in forcing the exchange beyond 95%. It has just been observed that this is only true for the process performed under relatively high partial pressures of CO2: a very substantial gain in efficacy can be obtained for the decarbonatation under low partial pressures of CO2, of about 2 mbar, with zeolites LSX in which the degree of exchange with sodium (defined as the molar ratio between the sodium ions and the aluminium atoms in a tetrahedral position, the remainder being potassium) is at least 98%.